All cells respond to changes in their immediate chemical environments by using signal transduction pathways that convey information from cell-surface receptors to specific intracellular signaling proteins. Fundamental processes ranging from neural transmission and immune responses to bacterial pathogenesis rely on such pathways. Defining how they operate at a molecular level is an important goal in many fields of modern biomedical research. In many systems, signal transduction is accomplished by complexes of membrane-spanning receptors, protein kinases, and 'adapter proteins'that hold together the receptors and kinases in appropriate combinations. There are many fundamental, unanswered questions about such signaling complexes: How do they assemble and disassemble? How do they achieve the necessary speed and sensitivity? How do adapter proteins contribute to the assembly and functioning of the complexes? The long-term objective of this project is to answer these questions for one particular sensory-response pathway (the chemotaxis system of E. coli), with the ultimate goal of defining general principles of signal transduction that can be applied to other systems. The bacterial chemotaxis system is one of the most intensively studied and most tractable sensory-response systems in nature. It allows cells to control their swimming movements in response to minute concentration changes in a variety of chemicals. These responses are mediated by a set of receptor proteins that interact with a protein kinase (CheA) and an adapter protein (CheW), forming complexes referred as 'signal teams'that can form larger assemblies ('signal clusters'). The molecular mechanisms underlying the assembly and cooperative abilities of signal teams and clusters have not been elucicated, but are known to require CheW and to involve interactions with CheB. We will investigate the mechanisms underlying sensitive, cooperative responses with the following Specific Aims: (1) Define the contributions of the adapter protein, CheW, by examining the actitivies of teams/clusters that incoroporate mutant versions of CheW;(2) Investigate how CheB contributes to detection sensitivity;(3) Investigate assembly, disassembly, and subunit movements within/between teams (4) Investigate interaction of signal teams and clusters with a newly discovered protein, CheV, that has intriguing similarties to CheW. Our results will help to define general concepts about regulation and clustering of receptor-kinase complexes.